The present invention relates to composite particles which represent substantially improved modifications of inorganic and organic pigments and which, depending on the pigment material, act as absorbers for visible and/or infrared light and can therefore be used as color-imparting agents and/or as infrared (IR) light absorbers and give rise to a negligible haze in the matrix.
The composite particles according to the invention can be used in media in which a high transparency, in other words low haze, is important, such as e.g. in clear varnishes, paints, plastics, glass or coatings made from these materials. They can also be used to adjust color tones in non-transparent media.
It is known that the size of inorganic pigments that are used in transparent systems should be below 10 nm or only a few tenth of nm in order not to give rise to additional haze. Given their generally lower refractive index, this limit is set at somewhat larger particle sizes in the case of organic pigments, but here too excessively large pigments lead to haze. In addition, these small particlesxe2x80x94also known as xe2x80x9cnanoparticlesxe2x80x9dxe2x80x94must be very well dispersed in the matrix and stabilized, since an agglomeration of these particles, in other words the formation of large secondary particles, again leads to haze and often also to a change in color.
The disadvantage of the known technical solutions to this problem is the time and cost involved in incorporating the nanoparticles into the matrix in the necessary degree of dispersion. The particles, which are generally present in the form of powders or pastes, are subjected to intensive shear forces (e.g. by grinding) in order to break up the large agglomerates that are present into such small units that the scattering of light by these units and hence also the haze of the matrix (e.g. surface coating binder) becomes negligible. Breaking up agglomerated nanoparticles into isolated primary particles generally requires a substantially greater effort than is the case with agglomerates consisting of larger primary particles ( greater than 100 nm), for example. However, since the practically complete deagglomeration of the nanoparticles is absolutely essential for a transparent coloration, the poor dispersibility of conventional, transparent pigments becomes a very substantial disadvantage for their use.
The distribution of UV-absorbing inorganic particles in or on dielectric particles measuring around 300 nm in size, which in turn are incorporated into the matrix as supports, to ensure the transparency of the matrix when UV light-absorbing pigments are used in cosmetics, has been reported in WO 95/09895. The material from which the large dielectric particles are formed is chosen according to the matrix used such that the refractive index of the compound particle consisting of dielectric particle and pigment differs only marginally from the refractive index of the surrounding matrix, thereby maximising transmission in the entire visible part of the light spectrum. This publication discloses only UV-absorbing particles such as e.g. TiO2 or ZnO, however. The objective here was to obtain as high an optical transmission as possible in the wavelength range above 400 nm, which inevitably excludes colored pigments and solar IR absorbers.
Most attempts to improve the transparency and incorporability of transparent pigments target the dispersibility of the pigments. For instance, it was reported that an improved dispersibility of fine haematite particles could be achieved by means of an organic (JP-A 07 126 018) or inorganic (JP-A 05 208 829) treatment of the pigments or coating of the pigment particle surface. By improving the compatibility between the particle surfaces and the matrix or by shielding the adhesive forces between the pigment particles, the effort required for dispersion can be somewhat reduced. Nevertheless, all of these proposed approaches still require a very laborious dispersion down to almost primary particle size in order to achieve a transparent, intensive coloration. In fact, in the secondary treatment of iron oxide haematite particles with, amongst other things, silicon dioxide (EP-A 0 997 500), a SiO2 proportion of greater than 20% is expressly deemed to be unreasonable because the two desired effects of ageing resistance and improved dispersibility are already achieved with significantly less secondary treatment material.
The production of supported particles, which can also be colored, is reported in connection with their use as heterogeneous catalysts (Catal. Today (1997), 34, 281-305). Iron oxide haematite applied to silicon dioxide for use as a heterogeneous catalyst is also described (React. Kinet. Catal. Lett. (1999), 66,183-188). There is no mention of a possible use as a transparent pigment, however, and the transfer of catalyst properties to optical pigment properties in a polymer or paint matrix is in no way obvious.
In order to synthetically adjust a mineral, thivier""s earth, a pressed cake of iron oxide particles in the goethite crystal modification was incorporated into a silicon dioxide dispersion (EP-A 0 947 564). However, the objective of this work was a composite in which the particle size of the iron oxide preferably has a value of between 0.1 and 1 xcexcm. This resulted not in a transparent color but ratherxe2x80x94as is immediately apparent to the person skilled in the artxe2x80x94a scattered color. These pigments would therefore be unsuitable for the transparent coloration of organic matrices.
An object underlying the present invention was therefore to provide specially modified highly transparent composite particles which absorb visible and/or infrared light and do not exhibit the disadvantages known in the prior art. Infrared (IR) light refers here to light that in solar radiation lies on either side of the visibility limit, i.e. in the wavelength range between approx. 700 nm and approx. 2500 nm.
The invention relates to a composite particle comprising a solid inorganic or organic colorless support material and adherent to it an inorganic and/or organic pigment particle, wherein the pigment particle has an average particle size of 1 nm to 100 nm, comprises at least one primary pigment particle and the minimum average distance between two pigment particles is at least one quarter of the diameter of the primary pigment particle.